1. Field of the Invention
At on-site oil production locations, hydraulically operated bottom-hole piston pump failure is frequently the result of excessive wear in the valves of the pump and, more frequently in, the precision shuttle valve which controls the motion of the engine piston. The drive fluid to the engine shuttle is often supplied from reconditioned produced reservoir fluids, which can provide minimum lubrication. Since the pistons change stroke directions tens of millions of times a year, the resulting loss of valve integrity causes pump inefficiency, production fall-off, and even shut down.
In response to this problem, long stroke, double displacement piston pumps have been developed. Such long stroke pumps can provide the same volumetric output as the more rapid stroking short pumps but with fewer stroke reversals and less valve wear. However, the simplest double displacement long stroke pumps normally require that the engine piston and pump piston have the same diameter since engine and pump comprise the opposite sides of a single piston.
Under certain circumstances, it is necessary to change the ratio of the pump piston area to engine piston area (referred to as the pressure ratio), particularly when designing pumps for use in deep, low pressure wells. With pumps intended for such wells, it is often necessary to provide a small pump piston relative to the engine piston to increase the driving force of the pump. Conversely, for shallow and/or high pressure wells, it is frequently desirable to increase the area of the pump piston in order to provide for higher volumetric throughput.
To design for pressure ratios less than 1.0 or greater than about 1.15 it is necessary to axially separate the engine and pump pistons in individual cylinders. The pistons are then coupled by a connector rod. In one type of unit, a different diameter pump piston and cylinder than the engine is attached to each end of a double acting engine. The unit becomes very long because of the addition of pump cylinders to each end. Shortening all axial dimensions proportionately for proper fit in the well bottom assembly ultimately compromises the long-stroke feature. In a second type of unit, the engine is located entirely above the pump, and the piston rod alternately undergoes tension and compression depending on the direction in which the pistons are being driven. This type is designed with a short middle rod connector to prevent buckling when it is undergoing an axial compressive force. This unit also has a short stroke.
It would therefore be desirable to provide a single pump design in the shortest possible body, which allows both flexible pressure ratio design as well as a truly double displacement long stroke capability intended for the largest displacement possible. In particular, it would be desirable to provide a double displacement piston pump having an engine piston and pump piston coupled by a connector which is resistant to buckling regardless of its length.
For example, in a well with a lift requirement of 18,000 feet, the required pressure ratio is about 0.5. For a 21/2 inch free pump, the number of piston strokes required to produce a barrel of crude oil at these depths ranges from 330 to 733 Strokes per Barrel Liquid (SPBL) depending on the type of industrial pump used. By calculating the displacement of the subject invention in those same length pump bodies, only 162 to 231 SPBL are required. Thus, valve movement and related wear is reduced by about half to one third.
2. Description of the Prior Art
Commercial long-stroke piston pumps are available from the Guiberson Division of Dresser Industries, Houston, Tex. 77005 under the designations Type I and II; from Kobe Division of Trico Industries, Huntington Park, Calif. 92055, under the designation Type E; and from the National Production Systems, Division of National Supply Company, Los Nietos, Calif. 90610.